Pyridine-2,3-dicarboxylates are useful intermediates in the preparation of important herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts. Said herbicidal agents and methods for their preparation are disclosed in U.S. Pat. Nos. 4,798,619 and 4,758,667. Imidazolinyl nicotinates and derivatives thereof are highly effective herbicides at low rates of application and demonstrate selective control of noxious weeds in the presence of key economic crops and, further, exhibit exceptionally low mammalian toxicity.
Among the methods for preparing these useful pyridinedicarboxylate intermediates are those described in European Patent Application Publication Nos. 308 084 and 456 504. The importance of the pyridinedicarboxylate derivatives, particularly as essential intermediates in the manufacture of herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts, creates a significant need in the art for effective processes for their production.
Therefore, it is an object of this invention to provide a process for the preparation of a substituted or unsubstituted pyridinedicarboxylic acid ester via the single step condensation of an .alpha., .beta.-unsaturated oxime with an aminobutenedioate in the presence of an acid, optionally in the presence of a solvent.
Another object of the present invention is to provide a ready source of pyridinedicarboxylate compounds and derivatives of said compounds as key intermediates in the production of the herbicidal agents, 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts.
These and further objects of the invention will become more apparent by the description provided hereinbelow.